Tacrolimus is an immunosuppressive agent, approved by the FDA in 1990 for the prevention of rejection following liver transplantation. Tacrolimus is known to be metabolized by cytochrome P450 3A4 in the liver and intestine. Since tacrolimus suppresses the immune system, it predisposes patients to reactivation and/or disseminatin of tuberculosis. Rifampin is one of the first line agents for treatment of tuberculosis and known to be an inducer of cytochrome P450 3A4. Rifampin has been shown to cause clinically significant interactions with agents which are also substrates for cytochrome P450 3A4 such as cyclosporine. There is one case report in the literature reporting decreased tacrolimus levels with the concomitant administration of rifampin. This pilot study will evaluate the effects of rifampin on the pharmacokinetic disposition of tacrolimus.